The present invention relates to a method of analyzing nucleated red blood cells in a blood sample. More specifically the method analyzes blood cell distribution patterns obtained from two separate blood measurement results, determines the presence of nucleated red blood cells, and further enumerates nucleated red blood cells in a blood sample.
Normal peripheral blood contains mature red blood cells which are free of nucleus. Nucleated red blood cells (NRBCs), also known as erythroblasts, are immature red blood cells. They normally occur in the bone marrow but not in peripheral blood. A presence of a significant number of NRBCs in peripheral blood may reflect a disturbance of red blood cell maturation, such as megaloblastic anemia, hemolytic conditions including thalassemia, sickle cell crises, leukemia, and transfusion reactions. Therefore, it is of clinical importance to measure NRBCs. Traditionally, differentiation and enumeration of NRBC are performed manually. The process involves the smearing of a blood sample on a microscope slide and staining the slide, followed by manual visual analysis of the individual slide. The NRBC concentration is reported as numbers of NRBC per 100 white blood cells. Usually, 200 white blood cells and the numbers of NRBC present in the same region on a blood smear are counted and the numbers are divided by 2 to express the NRBC concentration as the numbers of NRBC/100 WBC. This approach is extremely time-consuming as well as being subjective to the interpretation of the individual analyzing the slide.
In recent years, several fluorescence flow cytometry methods have been developed for differentiating NRBCs. These methods utilizes specific nuclear staining technique to distinguish NRBCs from other cell types because it is difficult to differentiate NRBCs based on their electronic or optical properties.
U.S. Pat. No. 5,298,426 (to Inami et al.) discloses a fluorescence method for differentiating NRBCs. The method utilizes a two-step staining using a first fluid and a second fluid. Inami et al. teaches that the first fluid contains an erythroblast-staining dye that diffuses into nucleated red blood cells to specifically stain their nuclei, and then separating a group of NRBCs from other cell groups on a two-dimensional plot whereby the results of NRBC differentiation are computed. In order to differentiate leukocyte subpopulation concurrently, the first fluid also contains two additional fluorescence dyes, i.e., an eosinophil/basophil-staining dye and leukocyte-staining dye for specific staining of these cell types.
U.S. Pat. No. 5,559,037 (to Kim et al.) discloses a method for flow cytometric analysis of NRBCs and leukocytes. The method comprises lysis of red blood cells and NRBC cytoplasm from a whole blood sample to expose the NRBC nuclei to a vital nuclear stain and minimizing the permeation of the vital nuclear stain into the leukocytes and analyzing the sample by measuring fluorescence and two angles of light scatter. This method features a triple triggering method which blocks the signals from debris (fluorescent and non-fluorescent) and identifies the signals which fall below the ALL trigger but above the fluorescence trigger (FL3) as NRBCs. ALL is the axial loss of light or the light scatter signals detected at 0xc2x0 from the incident light. Therefore, pre-gating signals in more than one dimension are required in this method for identification of NRBC population. Since leukocytes are also nucleated cells, staining of these cells needs to be prevented to avoid interference to the fluorescence measurement. The preservation of leukocyte membrane and minimizing the permeation of the nuclear stain into the leukocytes are achieved by concurrently fixing the leukocytes with an aliphatic aldehyde during lysis of red blood cells. In addition, the method requires heating of the reagent to 42xc2x0 C. in order to obtain the NRBC and leukocyte differentiations.
U.S. Pat. No. 5,648,225 (to Kim et al) discloses a method of using a multipurpose lysing reagent for subclassification of nucleated blood cells. The method comprises the steps of lysing a blood sample with the multipurpose lysing reagent which contains a nuclear stain, incubating the sample mixture at an elevated temperature, and determining the nucleated blood cells including NRBCs with an automated electro-optical hematology instrumentation.
U.S. Pat. No. 5,879,900 (to Kim et al) discloses a method of differentiating NRBCs, damaged white blood cells (WBC), WBC and a WBC differential in a blood sample by flow cytometry. The method includes lysing a blood sample; staining NRBCs and any damaged white blood cells with a vital nuclear stain; analyzing the sample mixture by measuring at least one fluorescence, and at least one light scatter signals in a range from 0xc2x0 to 1xc2x0 and 3xc2x0 to 10xc2x0; constructing a three-dimensional plot from the fluorescence and light scatter signals; and differentiating and enumerating WBC, NRBC, damaged WBC and a WBC subclass differential.
EP 1 004 880 A2 discloses reagents and a method for discrimination and counting of nucleated red blood cells. The method includes the steps of lysing red blood cells, staining white blood cells and NRBCs, assaying the sample by measuring at least one scattered light parameter, and at least one fluorescence parameter.
U.S. Pat. No. 5,874,310 (to Li et al) discloses a method for differentiation of nucleated red blood cells. The method includes lysing mature red blood cells and analyzing the sample in a flow cell by light scatter measurement to differentiate NRBCs from other cell types. The light scatter measurement is performed by using two low angle light scatter signals of less than 10xc2x0. The method further includes a concurrent differentiation of white blood cells using electronic and optical analysis, wherein the electronic analysis is a DC impedance measurement.
U.S. Pat. No. 5,917,584 (to Li et al) discloses a method for differentiation of nucleated red blood cells. The method includes lysing mature red blood cells in a blood sample; analyzing the sample in a flow cell by two angles of light scatter measurement to differentiate NRBCs from other cell types, wherein the first light scatter signal is a low angle light scatter signal of less than 10xc2x0, and the second light scatter signal is a medium angle or a right-angle light scatter signal.
The above described methods enable differentiation and enumeration of NRBCs and leukocytes by fluorescence flow cytometry and multi-angle light scatter measurements. However, fluorescence and multi-angle light scatter measurements are complex and expensive detection methods.
Many current automated hematology analyzers, such as Abbott Cell-Dyne(copyright) 3500, COULTER(copyright) GEN*S(trademark), Bayer Advia*120(copyright), and Sysmex(trademark) NE-9000 are only able to provide NRBC flagging for the possible presence of NRBCs in an analyzed blood sample when the instruments sense an increased amount of signals near blood cell debris area of an obtained cell distribution histogram. However, these methods are prone to generate false positive flaggings because many other blood abnormalities can cause increased signals at the same area, such as platelet clumps and sickle cells, as well as cell debris from insufficiently lysed blood samples.
Furthermore, a known problem with NRBC containing samples is erroneous white blood cell count (WBC) reported by hematology analyzers on these samples. Since the nuclear volumes of NRBC are close to those of white blood cells, and they are commonly counted as white blood cells on hematology analyzers which measure the sizes of blood cells, resulting an elevation of WBC. Therefore, correction of NRBC contribution to the WBC reported from hematology analyzer is required for samples containing NRBCs. Current practice in clinical laboratory is to subtract the numbers of NRBC obtained by manual count from the WBC reported by the hematology analyzers. This is time consuming and error prone.
Therefore, there presents a need for a simple and less expensive measurement method for analyzing and enumerating NRBCs in a blood sample.